In situ retorting of oil shale using co2 as heat carrier



Nov. 25, 1969 H. E. GILLILAND 3,480,082

IN sITU RETORTING 0F OIL SHALE USING cozAs HEAT CARRIER Filed Sept. 25, 1967 fl? /|0 6 25 2| 23 24 25 Q6-m9, fr nooo l :y www 22 HRL E. G/LL/L/V BY United States Patent O U.S. Cl. 166--266 4 Claims ABSTRACT oF THE DISCLOSURE Carbon -dioxide is utilized as heat carrier for retorting oil shale which is intimately associated with carbonate minerals. The carbon dioxide has high heat capacity and with the high CO2 atmosphere prevents calcining the carbonate, thus conserving heat.

BACKGROUND OF INVENTION Field of invention This invention relates to recovery of oil from oil shale either in situ or by surface retorting. More particularly, this invention relates to conservation of energy in heating oil shale associated with a carbonate mineral to retorting conditions with a carrier fluid.

Oil shale deposits are found in many locations with very large reserves of hydrocarbons. The hydrocarbon material is generally present as semi-fluids to solids and generally referred to as kerogens. These hydrocarbons are valuable sources of liquid petroleum products and have added vastly to the potential petroleum reserves of the world. While in many deposits of kerogens the oil shale is associated with clayey materials and non-reactive mineral, there are many deposits of such shale closely associated with carbonates such as calcite and dolomite.

Description of prior art In recovering usable hydrocarbons from oil shales, many proposals have been made. In those methods most widely used, heat is involved which softens or liquees the kerogen and/or cracks such material to produce liquid and gaseous products. The heat can be applied in situ or the shale can vbe mined by conventional mining methods such as room and pillar mining, long-wall mining, strip mining or any other method for removing the oil shale from its natural environment, and the shale is then subjected to retorting. When in situ retorting is utilized, the product can be recovered from the same well through which the heating agent is injected; however, more generally, the heating agent is injected in one well and the product is produced through one or more wells spaced from the injection well. Sometimes air is injected into the well and the kerogen is ignited. The hot gases resulting from the combustion move through the formation liquefying and partially gasifying the kerogen and carry the liquid and gaseous product through the formation to the production well where it is recovered. This in situ combustion has the obvious disadvantage of consuming useful components of the kerogen along with the least desirable components or rather those particularly useful as fuels. To overcome this disadvantge, an inert gas such as steam, nitrogen, ue gas and the like is heated on lthe surface and forced into the formation, and the hydrocarbons are withdrawn at a production well; the low molecular weight gaseous material is separated, from the desired liquid product, and burned to heat the injected gas. One such method for recovering hydrocarbons from 34,480,082 Patented Nov. 25, 1969 ICC tar sands, rather than oil shale, is described in U.S. Patent 3,040,809 Pelzer. A controlled combustion system is disclosed in U.S. Patent 2,839,141 by Water. R. W. Thomas in U.S. Patent 3,284,281 discloses fracturing and heating with inert gases. While CO2 has been disclosed by these patentees either in admixture with other gases or alone, the CO2 has been equated with other inert gases and the prior art has failed to recognize the problem of carbonate-containing material being associated with the oil shale or any advantage of utilizing CO2 as the inert gas.

SUMMARY OF INVENTION According to this invention, oil shale in association with carbonate minerals (wherein I mean by carbonate minerals-calcite, dolomite, nahcolite, trona, etc.) are heated to a temperature wherein the hydrocarbon associated with said oil shale is liquefied or gasied by contacting such oil shale with hot carbon dioxide. The oil shale can be contacted in situ or can be mined and lcontacted in a retort.

BRIEF DESCRIPTION OF DRAWINGS FIGURE 1 is a schematic flow diagram illustrating the invention as applied to in situ treatment of oil shale.

FIGURE 2 is a schematic ow diagram illustrating the invention as applied to surface retorting of oil shale.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION As has been indicated, when oil shale is associated with carbonate minerals, certain problems are encountered which do not exist with other oil shales. Frequently the carbonate minerals will equal approximately 50 percent of the mixture of minerals and hydrocarbon material. At temperatures of approximately 1000 F. which are required to retort the shale for recovery of hydrocarbons, the carbonates begin to calcine, e.g., decompose, consuming prodigious quantities of heat which will generally range from about 600,000 to 1,000,000 B.t.u. per ton of shale. The cost of supplying this heat can range from two to four times or more than that required for supplying the minimum heat requirement for heating the shale to retorting temperatures. The carbonates in the shale are also one of the principal agents for bonding the mineral matrix 0f the shale. Thus, the strength of the shale is deleteriously aifected by carbonate decomposition. This loss of strength leads to particle size degradation and shale dust is formed. Particularly in surface retorting, this dust creates numerous operating problems and contributes to atmosphere polution. To a lesser degree, the dust is also a problem from in situ retorting since the dust is easily carried out through the producing well and also tends to block the ow paths in the subterranean formation.

The decomposition of the carbonate minerals can be substantially reduced if the mineral is heated to calcining temperature in an atmosphere of CO2. Even at atmospheric pressures CaCO3 decomposition is reduced by a factor between 3 and 4 as compared to the decomposition at calcining temperatures in a nitrogen atmosphere. As the partial pressure of the CO2 is increased, the amount and rate of decomposition is decreased; therefore, I prefer to use a pressurev of at least 500 pounds per square inch gauge (p.s.i.g.) and generally use a pressure of at least 1000 p.s.i.g. The upper limit on the pressure is limited only by the structural strength of the formation or retorting vessel. Since the retorting temperature is generally around 1000 F., the CO2 must be heated to some temperature in excess of the 1000 F., the minimum temperature depending upon the formation temperature, the volume of gas per volume ofl shale to be heated and other` heat loss factors which will vary with.each...given..set-ofconditions.: Y

The determination of such temperature is within the skill of the art utilizing well known physical and chemical principles. In general, the minimum practical temperature will be about 1250 F.`fand preferably about 1500 F. The maximum temperature" will depend upon the nature of lthe' kerogenv and the degree of -thermal Acracking desired and will `be limited'by materials technology. This, of course, is not part of 'my invention and the -determination of the desired temperature tor-whichr the shale is to be heated is'well-'within the sk ill lof the art;v

Theuseof CO2 as the heat carrierhas additional advantages'over otherfinert'gasesFor' example,fCO2has` a relativelyhigh heat'capacity and' therefore is an efficient heat' carrier. The vCO2 `is easily 'liquefied' attemperatures below=87-8 E., thus'facilitating` recoveryof the CO2 for recirculation and handling. l 4

Having set forth certain broad'lir'nits of the invention, its `application for in situ retorting and. for surface Vretorting will be described in preferred embodiments by reference to the drawings. Whilecertainspecific` volumes, temperatures and pressures will be used, it is obvious that these can be varied within the framework of the foregoing general disclosure.

Describing first an in situ retorting embodiment, reference is made to FIGURE 1. Carbon dioxide gas is passed via conduit 1 and well 2 to oil shale formation 3 at a rate of 1,000,000 s.c.f.h. (standard cubic feet per hour), a temperature of 1500 F. and at a pressure of 1000 p.s.i.g. The CO2 heats the shale and vaporizes and/or entrains the hydrocarbons produced by cracking and/or vaporization and liquefaction and is produced from the formation through production well 4 and passed via conduit 5 to heat exchanger 6. The temperature of the stream entering exchanger 6 is 300 F. and has a pressure of 750 p.s.i.g. The stream is cooled to about 200 F. and passes via conduit 27 to separator 7 where a first condensate is removed via conduit 8 and passed to product line 9 to be sent to storage, not shown. The gaseous material is then passed to a second heat exchanger 10 via conduit 11 where it isfurther cooled to about 100 F. Vand passed via conduit 12 to a second separator 13 where theremaining condensible liquid is separated from the gaseous material and is passed via 'conduit 14 to conduit 9 and mixed with the liquid from separator 7 to be passed to storage. The mixed product amounts to 1000 bbls./day oil and 150 bbls/day water. The water can, of course, `be separated from the oil by well known decanting or other separating means and is not shown. The CO2 and light hydrocarbon gases are then passed at 700 p.s.i.g. to CO2 liquefaction zone 16 via conduit 15. Make up CO2 is added via conduit 17. The CO2 is liquefied and is separated from the gaseous hydrocarbons. These hydrocarbons amount to 1400 s.c.f./bbl. of shale oil recovered and have an energy Value of about 800 B.t.u./s.c.f. This fuel is passed via conduit 18 to primary heating zone 26. Additional fuel is mixed with this recovered fuel via conduit 19. The liquid CO2 is passed via conduit 21 and pump 22 at 40 F. and 1200 p.s.i.g. to heat exchanger 10 where it is heated to 140 F. and passed via conduit 24 to heat exchanger 6 where the temperature is raised to 240 F. The CO2 then passes via conduit'25 to primary heater` 26 where it is heated by burning the fuel from conduit Y9 -with air supplied via conduit toa temperature of 1500 F., the pressure having dropped to 1000 p.s.i.'g. and is then passed via conduit 1 to injection well 2 as previously described.

Normally, with an' oil shale-carbonate deposit, one would expect a net make of CO2 particularly where the hot gases enter the formation. However, operating according to the preferred method described above, it vis necessary to add make up CO2`via` conduit 17.

In the above description, valves, details of the liquelfaction zone, the separators andthe like'have been i omitted, since'these are not'part of invention and can readily be supplied by those skilled in the-art. Itis obvious that the CO2 can be directly heated without passing through the heat exchangers and the product from well 4 can be cooled by separate means if desired. It is also obvious that the liquid and gaseous product from the shale-carbonate layer 3 could be produced from the same well into which the hot CO2 is injected or could be produced from more than a .single production well. All of this is within the skill of the art.

A preferred method of treating oil shale mixed with a carbonate ina. surface retorting operation will be described with reference to FIGURE 2. Raw shale feed containing about 45 percent dolomite plus calcite, after being reduced to the desired particle size, is fed via conduit 1 at 50 F. and at a rate of 1000 pounds per hour per square foot of retort cross-sectional area to vertical kiln 2 where it is contacted by a rising stream of hot CO2. Hot CO2, at 1500 F. and at a rate of 4780 s.c.f./ton of shale, `is introduced to vessel 2 via conduit 3 at the desired operating pressure, preferably at least 500 p.s.i.g. CO2 at 150 F. and a rate of 20,900 s.c.f./ton of shale oil is introduced at the bottom of vessel 3 via conduit 4 and Serves to cool the spent shale. The spent shale is removed from vessel 2 via conduit 5 at 250 F. and sent to storage or dump as desired (not shown).

The hot gases (CO2 and hydrocarbon) with entrained liquid hydrocarbon passes overhead from vessel 2 via conduits 6 and 7 at a temperature of about 130 F. wherein most of the hydrocarbons liquefy and are passed to separator 8 via conduit 9. The liquid hydrocarbon product is removed from separator 8 via conduit 10 and sent to storage, not shown. The gases are cooled to about F. in separator 8 and passed via conduit 11 to zone 12 where the CO2 is liquefied and separated from the hydrocarbon gases. Make up CO2 is added via conduit 13. Cold CO2 is passed via conduit 14 to pump 15 and is introduced to vessel 2 as previously described. Hydrocarbon gases of 800 B.t.u./s.c.f. are passed at a rate of 1000 s.c.f./ton of oil shale processed via conduit 16 and with additional fuel gas from conduit 17 are burned in heater 18 with air supplied via conduit 19 to heat the CO2 to be passed to vessel 2 via conduit 3. The flue gas is taken off via conduit 20. The CO2 to be heated is passed via conduit 21, pump 22 and conduit 23 to Zone 18 where it is heated to 1500 F. and passed to vessel 2 via conduit 3 as described supra.

As in the case of in situ retorting, it is within the skill of the art to modify the flow described as required by the nature of the oil shale.

Having thus described my invention, I claim:

1. A process for recovering hydrocarbon product from a subterranean deposit of oil shale in association with a carbonate mineral, said process comprising introducing into said subterranean deposit a gas consisting essentially of CO2 at a pressure of at least 500 pounds per square inch gauge and at a temperature of at least 1000 F. wherein at least a portion of the deposit will be heated to a temperature above the ordinary decomposition temperature of said carbonate mineral in the absence of CO2, recovering from said subterranean deposit a mixture of CO2 and hydrocarbon and separating the recovered hydrocarbon from said CO2.

2. The process of claim 1 wherein the CO2 is separated from the hydrocarbon by liquefying the CO2.

3. The process of claim 2 wherein the CO2 is introduced into the subterranean deposit and the mixture of CO2 and hydrocarbon ishrecovered through the same well.

4. The process of claim 2 wherein the CO2 is introduced into the subterranean formation through a rst well, and the mixture of CO2 and hydrocarbon are recovered through a second well spaced from said iirst well.

Y, (References on following page) References Cited UNITED STATES PATENTS Day 166-8 Hoover et al. 166-7 McKee.

Deering et al 208-11 Friedman 20S-11 Natland 208-11 X Dougan 166-7 6 3,303,881 2/ 1967 Dixon. 3,342,257 9/196-7 Jacobs et al. 166-11 CHARLES E. OCONNELL, Primary Examiner 5 IAN A. CA-LVERT, Assistant Examiner U.S. Cl. X.R. 

